In 1972, geophysicist
Dan McKenzie was among the first to recognize that patterns of fault
block motion along the active zone of continental collision
in Eurasia are best explained in terms of rigid microplates that act
as dies or indenters. Indenters, such as Arabia, bulldoze the less
rigid (plastic)
crustal domains ahead into folded welts (e.g. Iran) and push some blocks
aside (e.g. Turkey). The geometry of deformation around indenters is
controlled by the shape of the impinging rigid face and to the boundary
conditions
of the surrounding plastic rocks at depth. As a working hypothesis,
Chamberlin and Anderson (1989) suggested that structural patterns in
the Laramide
Zuni uplift are much smaller but otherwise quite similar to indentation-extrusion
domains observed between India and south China. We suggest that the
narrow NNE-trending El Morro gravity high represents a relatively rigid
mafic
crustal beam, a stiletto heel, that focused compressive stresses and
progressively pushed up the core of the Zuni Mountains at its hard
northern tip. Slip
patterns of microthrusts on the NE flank of the Zuni uplift imply that
the core of the uplift moved northward with time (Chamberlin et.al,
1989; Chamberlin and Anderson, 1991). These limited observations provide
some
support for the indentation hypothesis. More field work is needed to
address the slip characteristics inferred zones of lateral shear on
the north and
west flank of the uplift . The concept that large rigid crustal blocks
or microplates can control patterns of deformation in surrounding plastic
rocks is also useful in viewing extended terranes in the Cordillera
of western North America. With this concept, the Colorado Plateau,
Idaho Batholith
and Sierra Nevada-Great Valley microplates appear as large rigid rafts
surrounded by a sea of westward drifting crustal slivers that resemble
pack ice.

Photo:
Stiletto heel mimics hypothetical indentation process along the
south flank of the Zuni Mountains.